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Assessing the Threat

To predict the effects of bioweapons, we need more data.

Could terrorists, intent on causing as much harm and societal disruption as possible, use new biotechnology processes to engineer a virulent pathogen that, when unleashed, would result in massive numbers of dead? Mark Williams, in his article “The Knowledge,” suggests we should be contemplating this doomsday scenario in the 21st century. Williams’s article might make you sleep less soundly, but are the threats real? The truth is that we do not really know.

Part of the problem is that even if terrorists could create new pathogens virulent to humans, it’s not at all clear that they could “weaponize” them – that is, put the pathogens into a form that is highly infectious to humans and then disperse them in ways that expose large numbers of people.

Past experience suggests that this is not an easy task. During World War II, the Japanese dropped plague-infected materials on Chinese cities, to limited effect. In 1979, the Soviets caused 66 deaths from anthrax by accidentally releasing it from a bioweapons facility in Sverdlovsk. In 1984, the Rajneeshees cult contaminated salad bars in the Dalles, OR, with salmonella, but their actions killed no one. In 1993, the Aum Shinrikyo cult failed to kill anyone after carrying out multiple attacks with anthrax in Japan. Finally, the 2001 anthrax letter attacks in the U.S. killed five people. These were all frightening events. They were not, however, grave threats to national security.

Yet estimates of bioweapons dangers tend to be dire, like those in Williams’s article. The truth is that the data are too thin to make accurate projections of the effects of bioweapons attacks. I surveyed seven separate estimates of fatalities from a projected anthrax attack. The lowest estimate, by Milton Leitenberg, ranged from zero to 1,440 dead per kilogram of anthrax used, while the highest, by Lawrence Wein and others, put fatalities between 123,400 and 660,000 per kilogram of anthrax. Most of these estimates were made on the basis of little actual data.

To predict accurately the effects of bioweapons, data are needed on the amount of agent required to infect a person, the percentage of people who survive an infection (which depends on the health of the population), the transmission rate if the agent is contagious, the ability to aerosolize and disperse an agent effectively (which depends, in turn, on climatic conditions), the environmental stability of an agent, the population density, and the abilities of the public-health system, including when an attack is detected and whether prophylactics, vaccines, or antidotes exist and, if so, in what quantities.

For any one pathogen – even one familiar to us, like smallpox and anthrax – not all of these variables are known, and therefore quantitative predictions are not possible with a high degree of certainty. In the words of the U.S. National Academy of Sciences in a 2002 report, “these factors produce an irreducible uncertainty of several orders of magnitude in the number of people who will be infected in an open-air release.”

For example, data on the infectiousness of an agent varies widely, depending on the agent. Because of limited experience with anthrax, susceptibility data have often been extrapolated from animal trials that have little bearing on human response to agents. In the case of smallpox, with which scientists had much experience in the 20th century, some factors remain uncertain, such as the transmission rate.

In the models of bioweapons attacks, the ability to weaponize an agent and disperse it effectively is estimated in part from open-air trials done by the U.S. Army between the 1940s and 1960s. These trials used live simulants of agents on major U.S. cities, but the behavior of a real bioweapon agent in such a situation remains uncertain. Williams’s article doesn’t describe in any detail the ability of terrorists to weaponize any of the theorized agents. Yet making effective bioweapons would take a tremendous amount of work.

While a state-sponsored program might have the means to do that work, terrorist groups probably don’t. With so much uncertainty surrounding the outcome of a bioweapons attack, it does not make sense to plan extensive biodefense programs when more-certain threats, particularly those involving nuclear weapons, require attention.

Allison M. Macfarlane is a research associate in the Science, Technology, and Global Security Working Group in MIT’s Program in Science, Technology, and Society.

Home page illlustration by Otto Steininger.

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